Biomedical Engineering Reference
In-Depth Information
Tensile
load
Load
Coating
Al rod
Epoxy resin
Coating
Substrate
Substrate
Epoxy resin
Jig
Load
Tensile
load
(a)
(b)
FIGURE 1.6
Schematic diagrams of pull-out testing methods for assessing of adhesion strength: (a) tensile adhesion strength
test (from Zhang et al.,
Thin Solid Films
, 516, 16, 5162-5167, 2008; with permission); (b) shear adhesion strength
test.
Hydroxyapatite—Part 4: Determination of Coating Adhesion Strength 2002). Both of these
two methods are tested using epoxy or super glues to fix the coating onto the counterpart.
Accordingly, the bonding strength will be highly influenced by many factors, such as the
uniformity of the epoxy layer, penetration depth of the glues, porosity and thickness of the
coating, and alignment of the applied force. Therefore, it could be imagined that a wide
range (large deviation) of adhesion strengths would be obtained, even for the coatings
deposited with the same deposition method. According to the documented results, the
adhesion strengths of HA coatings on metallic substrates prepared with different deposi-
tion techniques were summarized in Table 1.5. As stipulated by ISO standards (ISO13779),
the recommended bonding strength (pull-out tensile test) should be not less than 15 MPa
(Implants for Surgery-Hydroxyapatite—Part 2: Coatings of Hydroxyapatite 2000). The sol-
gel method is thus acceptable for the preparation of HA coatings from the standpoint of
adhesion (cf. Table 1.5).
More importantly, the adhesion strength after
in vitro
or even after
in vivo
tests is quite
crucial in estimating the survivability of HA coating on implant surfaces. As reported,
in vivo
studies suggested that the failure of plasma sprayed HA-coated metallic implants
mainly occurs at the coating-substrate interface, and the failure probability at this inter-
face increased with the period of implantation because the strength of the coating-bone
interface tends to increase with more healing time (Sun et al. 2001; Albrektsson 1998). Such
degradation of adhesion is generally attributed to the dissolution behaviors related to the
coating properties (e.g., impurity phases [TCP, CaO, etc.], crystallinity, cracks) (Lima et al.
2005), which would impair the mechanical properties (especially, adhesion strength) of the
coating. Detailed description and discussion about the adhesion properties after
in vitro
and/or
in vivo
tests can be found in some related reports (Aksakal, Gavgali, and Dikici
2009; Zhang et al. 2008; Kim et al. 2005; Albrektsson 1998).
Noticeably, cohesion failure (failure within the coating layer) was always observed in
those pull-out based tests (Figure 1.7), indicating that the obtained results are not the actual
